Golf ball

ABSTRACT

A golf ball ( 1 ) comprises a center ( 2 ), a rubber thread layer ( 3 ) and a cover ( 4 ). The rubber thread layer ( 3 ) is formed by winding a drawn rubber thread around the center ( 2 ). The rubber thread is obtained by cutting a crosslinked rubber sheet. A hysteresis loss of the crosslinked rubber sheet is 30% or less with an extension range of 0% to 1000%. Moreover, a universal hardness of a DIN standard measured with a load of 3 mN is 1.5 N/mm 2  to 5 N/mm 2  when a whole length is increased up to 1000%. The rubber thread is broken with difficulty during drawing. The golf ball ( 1 ) has an excellent resilience performance.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a golf ball and more particularly to a wound golf ball having a wound core.

[0003] 2. Description of the Related Art

[0004] A round golf ball to be used for a play in a golf course is roughly divided into a wound golf ball having a core in which a rubber thread is wound and a solid golf ball (two pieces golf ball, three pieces golf ball and the like) having a core formed of only a solid rubber. In general, the wound golf ball is excellent in a hit feeling and a control performance and the solid golf ball is excellent in a flight distance and a durability. The wound golf ball has been used for a long time. For a certain period, almost all first-class golf balls were represented by the wound golf balls. The solid golf ball developed later can be manufactured easily at a low cost. In recent years, therefore, more solid golf balls than the wound golf balls have been put on the market.

[0005] Under the circumstances, a professional golf player and an advanced amateur golf player still require the wound golf ball excellent in the hit feeling and the control performance. The professional golf player and the advanced amateur golf player have desired the wound golf ball providing a flight distance equivalent to that of a solid golf ball. Moreover, many general amateur golf players want to use the wound golf ball if the flight distance is increased.

[0006] The golf ball is launched and flies by hitting with a golf club. In general, if the speed (initial speed) of the golf ball is higher immediately after the hitting, a flight distance tends to be increased. Accordingly, it is possible to lengthen the flight distance of the wound golf ball by increasing the initial speed, that is, a resilience coefficient.

[0007] An ordinary wound golf ball includes a core and a cover. The core is constituted by a center (a solid center or a liquid center) and a rubber thread layer having a rubber thread drawn and wound around the center. The rubber thread layer most contributes to the resilience performance of the wound golf ball. An attempt to enhance the resilience performance of the wound golf ball has been made by improving the rubber thread to be used for the rubber thread layer for a long period of time. For example, Japanese Patent Publication No. Sho 61-12706 (1986 /12706)has disclosed a wound golf ball in which carbon black is used for a rubber composition constituting a rubber thread. Moreover, Japanese Patent Publication No. Hei 5-41272 (1993 /41272) has disclosed a wound golf ball in which a specific base rubber is used for a rubber composition constituting a rubber thread.

[0008] In respect of an enhancement in the resilience performance of the golf ball, the rubber thread has a draw ratio of 800% to 1100%. Thus, the draw ratio is very high. Therefore, the rubber thread is often broken in a winding stage if a strength thereof is poor. Consequently, the productivity of the golf ball is deteriorated. In the extreme case, the core cannot be manufactured.

[0009] Thus, examples of the important performance required for the rubber thread include a resilience performance, and furthermore, a strength. However, these cannot be easily compatible with each other. For example, it is possible to propose means for preventing crystallization from being caused by drawing, thereby reducing a hysteresis loss and enhancing the resilience performance of the rubber thread. However, the rubber thread having the crystallization prevented is easily broken during the drawing. The resilience performance and the strength in the rubber thread are inconsistent with each other.

[0010] In consideration of such problems, it is an object of the present invention to provide a golf ball using a rubber thread which is broken with difficulty during drawing and having an excellent resilience performance.

SUMMARY OF THE INVENTION

[0011] In order to achieve the object, the present invention provides a golf ball comprising a center, a rubber thread layer formed by winding a rubber thread around the center, and a cover, wherein the rubber thread is formed of a material in which a hysteresis loss is 30% or less with an extension ratio of 0% to 1000% and a universal hardness of a DIN standard measured with a load of 3 mN is 1.5 N/mm² to 5 N/mm² when a whole length is increased up to 1000%.

[0012] Since the rubber thread formed of a material having a hysteresis loss of 30% or less with a draw ratio of 0% to 1000% is used for the rubber thread layer, the golf ball has an excellent resilience performance. Since the rubber thread is formed of a material in which a universal hardness of a DIN standard measured with a load of 3 mN is 1.5 N/mm² to 5 N/mm² when a whole length is increased up to 1000% (that is, an extension ratio is 900%), the rubber thread is broken with difficulty during drawing and winding.

[0013] It is preferable that the rubber thread should contain synthetic polyisoprene as a base rubber. A ratio of the synthetic polyisoprene to the whole base rubber is 70% by weight or more. The synthetic polyisoprene is a rubber containing trans 1, 4 bond. The trans 1, 4 bond suppresses the crystallization of a rubber molecule. By using the synthetic polyisoprene, the hysteresis loss of the rubber thread material is reduced so that the resilience performance of the golf ball can be enhanced. More preferably, synthetic polyisoprene having a ratio of the trans 1, 4 bond of 2% or more is used.

[0014] In the case of a rubber thread crosslinked with sulfur and a vulcanization accelerator, it is preferable that a weight ratio of the sulfur and the vulcanization accelerator should be 0.8/1 to 8/1. Consequently, a ratio of polysulfide bond occupying the whole amount of rubber molecule bond is increased so that the crystallization can be suppressed more greatly when the rubber thread is drawn.

[0015] It is preferable that across linking density of polysulfide bond in the rubber thread should be 1×10⁻⁸ mol/mm³ to 3×10⁻⁸ mol/mm³. Consequently, the strength of the rubber thread can be enhanced so that the rubber thread can be prevented from being broken during the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a perspective view showing a golf ball according to an embodiment of the present invention,

[0017]FIG. 2 is a perspective view showing a rubber thread to be used for a rubber thread layer of the golf ball illustrated in FIG. 1, and

[0018]FIG. 3 is a typical graph showing a result of a tensile test.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.

[0020]FIG. 1 is a perspective view showing a golf ball 1 according to an embodiment of the present invention. The golf ball 1 comprises a center 2, a rubber thread layer 3 and a cover 4. The golf ball 1 comprises a mark layer and a coated layer which are provided on the outside of the cover 4, and these layers are not shown in FIG. 1. Moreover, the golf ball 1 has dimples on the surface of the cover 4 and the dimples are not shown in FIG. 1.

[0021] The center 2 is a sphere and is a so-called solid center which is constituted by crosslinking a rubber composition. The type of a base rubber to be used for the center 2 is not particularly restricted but polybutadiene, a natural rubber or the like can be used. The center 2 usually has a diameter of approximately 20 mm to 35 mm. In place of the solid center, it is also possible to use a so-called liquid center in which a liquid or a paste-like fluid is filled in a hollow spherical bag formed of a crosslinked rubber.

[0022] The rubber thread layer 3 is formed by winding a rubber thread drawn around the center 2. The rubber thread layer 3 includes a rubber thread and a slight gap. During the winding, the rubber thread has a draw ratio of approximately 800% to 1100%. The rubber thread layer 3 usually has a thickness of approximately 1.5 mm to 15 mm. A core 5 is constituted by the center 2 and the rubber thread layer 3.

[0023] The cover 4 is provided on the outside of the rubber thread layer 3 in close contact with the rubber thread layer 3. The material of the cover 4 is not particularly restricted but a resin composition using a synthetic resin such as an ionomer resin as a base material or balata is suitably used. The cover 4 usually has a thickness of approximately 1 mm to 4 mm.

[0024]FIG. 2 is a perspective view showing a rubber thread 6 to be used for the rubber thread layer 3 of the golf ball 1 illustrated in FIG. 1. The rubber thread 6 formed of a material having a hysteresis loss of 30% or less with an extension ratio of 0% to 1000%. By using the rubber thread 6, the transmission efficiency of a kinetic energy is increased when the golf ball 1 collides with a golf club. Consequently, the resilience performance of the golf ball 1 can be enhanced. In particular, it is preferable that a material having a hysteresis loss of 28.0% or less, and furthermore, 26.0% or less should be used. In respect of the resilience performance, a smaller hysteresis loss is preferable. However, a rubber material having a hysteresis loss of zero is obtained with difficulty. Moreover, if the hysteresis loss is extremely small, the strength of the rubber thread 6 is reduced. Therefore, it is preferable that the hysteresis loss should be 15% or more, particularly, 20% or more, and furthermore, 22% or more.

[0025] The hysteresis loss is measured in the stage of a crosslinked rubber sheet which has not been cut into the rubber thread 6. In the measurement of the hysteresis loss, first of all, a rubber sheet is punched into a shape of a No. 4 dumbbell defined in JIS. Thus, a specimen is obtained. The specimen is punched out such that the direction of the length of the dumbbell is coincident with that of the rubber thread 6. The specimen is attached between upper and lower chucks of a tensile testing machine (205 model produced by Intesco Co., Ltd.), and is pulled at a crosshead speed of 500 mm/min. When an extension ratio reaches 1000%, a crosshead is returned to an original position at a speed of 500 mm/min.

[0026]FIG. 3 is a typical graph showing a result of the tensile test. In FIG. 3, a curve C1 indicates a relationship between a stress and an extension ratio (strain) during pulling. Moreover, a curve C2 indicates a relationship between a stress and an extension ratio (strain) during restoration. Furthermore, a line L indicates that the extension ratio is 1000%. In FIG. 3, the area of a region surrounded by the curve C2, the line L and an axis of abscissa is represented by S1. Moreover, the area of a region surrounded by the curves C1 and C2 is represented by S2. A hysteresis loss (Hy) is calculated by the following equation (I).

Hy=(S 2/(S 1+S 2))×100  (I)

[0027] The rubber thread 6 shown in FIG. 2 is formed of a material having a universal hardness of 1.5 N/mm² to 5 N/mm². The rubber thread 6 has a hysteresis loss of 30% or less as described above. By thus reducing the hysteresis loss, the rubber thread 6 tends to be easily broken during drawing. By setting the universal hardness of 1.5 N/mm² to 5 N/mm², it is possible to obtain the rubber thread 6 having a low hysteresis loss which is broken with difficulty. In other words, a resilience performance is compatible with the difficulty of breakage in the rubber thread 6.

[0028] If the universal hardness of the rubber thread 6 is less than 1.5 N/mm², the tensile strength of the rubber thread 6 is reduced so that the rubber thread 6 is easily broken during the drawing in some cases. In this respect, it is preferable that the universal hardness should be 2.1 N/mm² or more, particularly, 2.9 N/mm² or more. If the universal hardness is greater than 5 N/mm², an elongation at break (EB) is reduced so that sufficient drawing is hard to obtain during winding. In some cases, moreover, the rubber thread 6 is broken due to forcible drawing. In this respect, it is preferable that the universal hardness should be 4.0 N/mm² or less, particularly, 3.5 N/mm² or less.

[0029] The universal hardness is measured by “Fisherscope H-100” produced by Fisher Instruments Co., Ltd. in accordance with the DIN standard (DIN50395). The rubber thread 6 or a crosslinked rubber sheet which has not been cut into the rubber thread 6 is used as a specimen for the measurement. For the measurement, first of all, the specimen is extended to have a whole length of 1000 %. Consequently, the specimen has a thickness of approximately 0.2 mm. Next, a pyramid-shaped Vickers indenter formed of diamond is pressed from the surface of the specimen. When a load during pressing is represented by F(N) and a pressing depth is represented by h (mm), a universal hardness Hu (N/mm²) is calculated by the following equation (II). The load (F) during the pressing is set to 3 mN and a test atmosphere is set to 23° C.

Hu=F/(26.43 ×h ²)  (II)

[0030] When the rubber thread 6 is drawn, a rubber molecule is oriented in the direction of the drawing so that crystallization is generated. In the rubber thread 6 having a high degree of the crystallization, the hysteresis loss exceeds 30% due to the crystallization so that the resilience performance of the golf ball 1 is deteriorated in some cases. By using a rubber causing the crystallization with difficulty, the hysteresis loss can be set to 30% or less. Examples of the rubber causing the crystallization with difficulty include synthetic polyisoprene. The synthetic polyisoprene contains trans 1, 4 bond. Since the trans 1, 4 bond has a three-dimensional structure, the crystallization during the drawing of cis 1,4 bond is prevented. Another rubber such as a natural rubber or polybutadiene may be used together with the synthetic polyisoprene. Also in the case in which they are used together, it is preferable that a ratio of the synthetic polyisoprene occupying the whole base rubber should be set to 70% by weight or more, particularly, 80% by weight or more, and furthermore, 90% by weight or more.

[0031] In particular, the synthetic polyisoprene containing more trans 1, 4 bond (2% or more) is preferable in respect of the prevention of the crystallization. More specifically, it is preferable that the ratio of the trans 1, 4 bond contained in the synthetic polyisoprene should be 2% to 4%. In some cases in which the ratio is less than 2%, the effect of preventing the crystallization during the drawing is insufficient. If the ratio exceeds 4%, the elasticity of the rubber thread 6 is reduced in such a stage that the drawing has not been carried out. Consequently, the resilience performance of the golf ball 1 becomes insufficient. Such synthetic polyisoprene containing more trans 1, 4 bond is polymerized with a lithium based catalyst. The ratio of the trans 1, 4 bond is measured by a well-known method through a nuclear magnetic resonance apparatus (NMR).

[0032] While the crosslinking configuration of the rubber thread 6 is not particularly restricted, sulfur crosslinking is usually employed, and furthermore, a vulcanization accelerator is used together. The type of the vulcanization accelerator to be used is not particularly restricted but a thiuram type vulcanization accelerators, a guanidine type vulcanization accelerators, a thiazole type vulcanization accelerators, a sulfenamide type vulcanization accelerators and the like can be used suitably.

[0033] It is preferable that the amount of sulfur should be 0.5 to 7 parts by weight for 100 parts by weight of a base rubber. In some cases in which the amount of the sulfur is less than 0.5 part by weight, the crosslinking density of the rubber thread 6 is reduced so that a tensile strength becomes insufficient. In this respect, it is particularly preferable that the amount of the sulfur should be 1 part by weight or more. If the amount of the sulfur is more than 7 parts by weight, an elongation at break (EB) of the rubber thread 6 is reduced so that sufficient drawing is hard to carry out during winding in some cases. In this respect, it is particularly preferable that the amount of the sulfur should be 4 parts by weight or less.

[0034] It is preferable that the amount of the vulcanization accelerator should be 0.3 to 3 parts by weight for 100 parts by weight of the base rubber. In some cases in which the amount of the vulcanization accelerator is less than 0.3 part by weight, the crosslinking density of the rubber thread 6 is reduced so that a tensile strength becomes insufficient. In this respect, it is particularly preferable that the amount of the vulcanization accelerator should be 0.5 part by weight or more. If the amount of the vulcanization accelerator is more than 3 parts by weight, an elongation at break (EB) of the rubber thread 6 is reduced so that sufficient drawing is hard to carry out during the winding in some cases. In this respect, it is particularly preferable that the amount of the vulcanization accelerator should be 2 parts by weight or less.

[0035] In the case in which the rubber thread 6 is crosslinked with the sulfur and the vulcanization accelerator, a rubber component has polysulfide bond (including disulfide bond) in which rubber molecules are crosslinked with two or more sulfurs and monosulfide bond in which the rubber molecules are crosslinked with one sulfur. A monosulfide bonding portion easily causes the crystallization if the rubber thread 6 is drawn. To the contrary, the polysulfide bonding portion causes the crystallization with difficulty by the drawing. Accordingly, if the ratio of the polysulfide bond to a total bonding amount is high, the crystallization is suppressed so that the hysteresis loss of the rubber thread 6 is reduced. The ratio of the polysulfide bond to the total bonding amount is preferably 70% or more, and particularly preferably 95% or more.

[0036] A ratio (R) of the polysulfide bond is calculated by the following equation (III):

R=ν_(P)/ν_(T)  (III)

[0037] wherein a total crosslinking density is represented by (ν_(T)) and a crosslinking density of the polysulfide bond is represented by (ν_(P)).

[0038] The crosslinking density (ν_(P)) of the polysulfide bond is calculated by the following equation (VI):

ν_(P)=ν_(T)−ν_(M)  (IV)

[0039] wherein a crosslinking density of the monosulfide bond is represented by (ν_(M)).

[0040] The crosslinking density is calculated in the following procedure. First of all, a cylindrical specimen having a diameter of 3 mm is punched out of a crosslinked rubber sheet which has not been cut into the rubber thread 6. The specimen is immersed in acetone at 20° C. for 24 hours so that oil and an antioxidant are extracted. The specimen obtained after the extraction is immersed and swollen, for 24 hours, in a solvent at 20° C. in which tetrahydrofuran (THF) and benzene are mixed with a weight ratio of 1:1. Next, the specimen is put in a TMA apparatus filled with the solvent at 20° C. in which tetrahydrofuran (THF) and benzene are mixed with a weight ratio of 1:1. A value of (τ₀/(1/α²-α)) is calculated based a relationship between a compression stress and a strain in the TMA apparatus. A numeric value thus obtained and various dimensions of the specimen are substituted for Flory's theoretical equation expressed in the following equation (V) so that the total crosslinking density (ν_(T)) of the rubber thread 6 is calculated. A test is carried our for three specimens and the results thus obtained are averaged. $\begin{matrix} {\gamma_{T} = {\frac{\gamma \quad e^{\prime}}{{V0}^{\prime}} = {\frac{\tau_{0}}{{RT}\left( {\alpha - \frac{1}{\alpha^{2}}} \right)}\sqrt[3]{\frac{1 - \varphi}{\left( {{Ls0}/{L0}} \right)^{3} - \varphi}}}}} & (V) \end{matrix}$

[0041] τ₀: stress=f/A0 [g/mm²]

[0042] f: stress [g]

[0043] γe: number of crosslinked points [number]

[0044] γe′: number of crosslinked points [mol]

[0045] K: Boltzmann constant 1.381×10⁻²³ [J/K]

[0046] R: gas constant 8.314 [J/mol·K]→R=kNa (Na:avogadro's

[0047] number=6.02×10⁻²³ mol⁻¹)

[0048] T: measuring temperature [K]

[0049] V0: total volume of specimen [mm³]

[0050] V0′: volume of pure rubber polymer=V0 (1-φ) [mm³]

[0051] φ: volume fraction of filler(volume of filler/total volume of rubber)

[0052] α: compressibility of specimen after swelling=Ls/LsO

[0053] L0: height of specimen before swelling [mm]

[0054] Ls: height of compressed and swollen specimen [mm]

[0055] Ls0: height of specimen after swelling [mm]

[0056] A0: area of end face of specimen before swelling [mm²]

[0057] A1: area of end face of specimen after swelling=A0 (LS0/L0) [mm²]

[0058] τ₀ can be calculated by the following equation. $\tau_{0} = {\frac{{RT\gamma}\quad e^{\prime}}{{V0}^{\prime}} = \sqrt[3]{\frac{\left( {{Ls0}/{L0}} \right)^{3} - \varphi}{1 - \varphi}\left( {\alpha - \frac{1}{\alpha^{2}}} \right)}}$

[0059] The measurement for calculating the crosslinking density (γ_(M)) of the monosulfide bond is carried out in the same manner as that of the crosslinking density (ν_(T)) except that a LiAlH catalyst is added to a 1:1 mixed solution of tetrahydrofuran and benzene to swell the specimen. Moreover, the crosslinking density (ν_(M)) is calculated in accordance with the equation (V). In this case, (ν_(T)) in the equation (V) is replaced with (ν_(M)).

[0060] It is preferable that the crosslinking density of the polysulfide bond in the rubber thread 6 should be 1×10⁻⁸ mol/mm³ to 3×10⁻⁸ mol/mm³. If the crosslinking density is less than 1×10⁻⁸ mol/mm³, the universal hardness is less than 1.5 N/mm² so that the rubber thread 6 is easily broken during drawing in some cases. In this respect, it is particularly preferable that the crosslinking density should be 1.5×10⁻⁸ mol/mm³ or more. If the crosslinking density is more than 3×10⁻⁸ mol/mm³, the universal hardness is more than 5 N/mm² and an elongation at break (EB) of the rubber thread 6 is reduced so that sufficient drawing is hard to carry out during winding. In this respect it is particularly preferable that the crosslinking density should be 2.5×10⁻⁸ mol/mm³ or less.

[0061] In the case in which the sulfur and the vulcanization accelerator are used together, it is preferable that a weight ratio (sulfur/vulcanization accelerator) should be 0.8/1 to 8/1. If the weight ratio is less than 0.8/1, the ratio of the polysulfide bond is reduced so that the crystallization of the rubber thread 6 cannot be prevented during the drawing in some cases. In this respect, the weight ratio is particularly preferably 1.2/1 or more, and more preferably, 1.5/1 or more. If the weight ratio is more than 8/1, the crosslinking density is increased and the elongation at break (EB) of the rubber thread 6 is reduced so that sufficient drawing is hard to carry out during winding. In this respect the weight ratio is particularly preferably 4/1 or less, and more preferably, 3.5/1 or less.

[0062] For a rubber composition constituting the rubber thread 6, zinc oxide or the like may be blended as an activator in addition to the sulfur and the vulcanization accelerator, and furthermore, a proper amount of a filler such as clay, a softening agent such as oil, an antioxidant, other additives or the like may be blended.

[0063] Preferably, the thickness of the rubber thread 6 is 0.35 mm to 0.6 mm, and more preferably, 0.4 mm to 0.55 mm. If the thickness is less than the above-mentioned range, thread cutting may be caused easily. If the thickness exceeds the same range, the rubber thread 6 is stretched with difficulty during winding. For this reason, the density of the rubber thread 6 in the core 5 is reduced in some cases. The golf ball 1 using the core 5 with the small density of the rubber thread 6 has a low hardness and a poor resilience performance. The thickness of the rubber thread 6 may be measured in the stage of a crosslinked rubber sheet for convenience.

[0064] The rubber thread 6 is obtained through a kneading step, an extruding step, a crosslinking step and a cutting step, for example. At the kneading step, base rubber, a crosslinking agent, an additive and the like are kneaded so that a rubber composition is obtained. An internal kneading machine such as a kneader or a Banbury mixer, an open roll and the like are used for the kneading.

[0065] At the extruding step, the rubber composition obtained at the kneading step is put into a cylinder of an extruder and is extruded from a die of a head portion. An opening of the die is slit-shaped and the rubber composition is extruded like a sheet (an uncrosslinked rubber sheet). By using the extruder, the thickness of the rubber thread 6 can be prevented from being varied. More preferably, a screw type uniaxial extruder is used. In the screw type uniaxial extruder, an orientation of the rubber is controlled. Therefore, the uncrosslinked rubber sheet less shrinks after the extrusion. Consequently, the thickness can be prevented from being nonuniform. In the screw type uniaxial extruder, it is preferable that the speed of rotations of a screw should be set to 3 rpm to 60 rpm, particularly, 5 rpm to 40 rpm in respect of the uniform thickness of the uncrosslinked rubber sheet, and furthermore, the uniform thickness of the rubber thread 6.

[0066] Preferably, an internal temperature of the cylinder of the screw type uniaxial extruder is 50° C. to 100° C., and more preferably, 60° C. to 90° C. By setting the internal temperature of the cylinder to this range, concavo-convex portions can be prevented from being formed on a surface of the uncrosslinked rubber sheet and scorching can also be prevented. Preferably, an internal temperature of the head portion is 80° C. to 110° C., and more preferably, 80° C. to 100° C. By setting the internal temperature of the head portion to this range, the concavo-convex portions can be more greatly prevented from being formed on the surface of the uncrosslinked rubber sheet and the scorching can also be prevented.

[0067] Preferably, the thickness of the extruded and uncrosslinked rubber sheet is 2 mm to 6 mm, and more preferably, 3 mm to 5 mm. If the thickness is less than the above-mentioned range, the scorching is sometimes caused due to the heat generation of the rubber composition. If the thickness exceeds the same range, a considerable reduction in the thickness is required at the next step. Consequently, the thickness of the rubber thread 6 becomes nonuniform in some cases.

[0068] The thickness of the uncrosslinked rubber sheet thus extruded is usually reduced through a roller processing or the like. In respect of productivity, it is preferable that a roller head extruder should be used and the extrusion and the roller processing should be carried out in a single pass.

[0069] At the crosslinking step, the uncrosslinked rubber sheet is crosslinked to be a crosslinked rubbersheet. It is preferable that the crosslinking should be carried out through a continuous crosslinking device. The continuous crosslinking device comprises a heat roller and a belt pressed in contact with the heat roller. The uncrosslinked rubber sheet is inserted between the heat roller and the belt. In this case, the uncrosslinked rubber sheet is pressurized and heated and is thus crosslinked. By using the continuous crosslinking device, a surface roughness of the crosslinked rubber sheet is reduced. Examples of the continuous crosslinking device include a Rote—Cure type device manufactured by Adamson Co., Ltd. in U.S.A., an AUMA type device manufactured by Berstorff Co., Ltd. in Germany and the like.

[0070] Preferably, a pressure in the continuous crosslinking device is 0.03 MPa to 1 MPa, and particularly preferably, 0.1 MPa to 0.3 MPa. If the pressure is less than the above-mentioned range, the surface roughness of the crosslinked rubber sheet is not sufficiently reduced in some cases. If the pressure exceeds the same range, a modulus of the rubber thread 6 might be increased.

[0071] It is preferable that a crosslinking temperature in the continuous crosslinking device should be 140° C. to 160° C. If the crosslinking temperature is less than the above-mentioned range, the crosslinking time should be increased so that productivity might be deteriorated. If the crosslinking temperature exceeds the same range, overcrosslinking is caused to degrade the physical property of the rubber thread 6 in some cases. The crosslinking time in the continuous crosslinking device is usually set to approximately 3 minutes to 20 minutes.

[0072] In the continuous crosslinking device, the uncrosslinked rubber sheet is always crosslinked while abutting on the heat roller. In the case in which the uncrosslinked rubber sheet is wound onto the roller in many plies and is crosslinked by a vulcanizer, a variation in the physical property is caused by a difference in a heat conductivity between an inside sheet and an outside sheet. Such variation is not caused in the continuous crosslinking.

[0073] The crosslinked rubber sheet is cut to have a predetermined width at the cutting step. A well-known cutter can be used for the cutting. Thus, the rubber thread 6 can be obtained.

EXAMPLES [Example 1]

[0074] 70 parts by weight of synthetic polyisoprene containing 3% of trans 1, 4 bond and 92% of cis 1, 4 bond (trade name of “IR309” produced by Shell Co., Ltd.), 30 parts by weight of a natural rubber (pale crepe), 1 part by weight of zinc oxide (trade name of “zinc oxide No. 1” produced by Sakai Chemical Industry Co., Ltd.), 1 part by weight of 2,6-di-tert-butyl-4-methylphenol as an antioxidant (trade name of “Nocrac 200” produced by Ouchi Shinko Kagaku Kogyo Co., Ltd.), 1 part by weight of 1,3-diphenylguanidine as a guanidine type vulcanization accelerator (trade name of “Nocceler D” produced by Ouchi Shinko Kagaku Kogyo Co., Ltd.), 0.5 part by weight of N-cyclohexyl-2-benzothiazoryl sulfenamide as a sulfenamide type vulcanization accelerator (trade name of “Nocceler CZ” produced by Ouchi Shinko Kagaku Kogyo Co., Ltd.), and 3 parts by weight of sulfur were put and kneaded in a kneader. Thus, a rubber composition was prepared.

[0075] The rubber composition was formed like a ribbon and was put in a cylinder of a roller head extruder. The rubber composition was extruded from a head portion including a die opening having a thickness of 4 mm and a width of 200 mm and was thinned by means of a roller. Consequently, an uncrosslinked rubber sheet having a width of 300 mm and a length of 30 m was obtained. A temperature in the cylinder was set to 70° C., a temperature in the head portion was set to 90° C., a temperature in the roll was set to 90° C., and the speed of rotations of a screw was set to 20 rpm.

[0076] The uncrosslinked rubber sheet thus obtained was continuously crosslinked by using the continuous crosslinking device (the Rote-Cure type device manufactured by the Adamson Co., Ltd. in U.S.A.). Thus, a crosslinked rubber sheet was obtained. A crosslinking temperature was set to 150° C., a pressure was set to 0.2 MPa and a crosslinking time was set to 5 minutes. The crosslinked rubber sheet was cut to have a width of 2 mm. Thus, a rubber thread was obtained.

[0077] On the other hand, 100 parts by weight of polybutadiene (trade name of “BR01” produced by Japan Synthetic Rubber Co., Ltd.), 5 parts by weight of zinc oxide (trade name of “zinc oxide No. 1” produced by Sakai Chemical Industry Co., Ltd.), 75 parts by weight of barium sulfate, 0.2 part by weight of N-cyclohexyl-2-benzothiazoryl sulfenamide as a sulfenamide type vulcanization accelerator (trade name of “Nocceler CZ” produced by Ouchi Shinko Kagaku Kogyo Co., Ltd.), and 9 parts by weight of sulfur were put and kneaded in a kneader. Thus, a rubber composition was prepared. The rubber composition was put in a mold including upper and lower parts having semispherical cavities respectively, and was crosslinked at 150° C. for 30 minutes. Thus, a center having a diameter of 32 mm was obtained. A rubber thread was drawn and wound around the center to have a draw ratio of 1000%. Thus, a core having a diameter of 39 mm was obtained.

[Examples 2 to 7 and Comparative Examples 1 to 3]

[0078] A core was prepared in the same manner as that in the first example except that the amount of a compounding agent was varied as shown in the following Table 1.

[0079] [Measurement of Hysteresis Loss]

[0080] A crosslinked rubber sheet which had not been cut was punched out to a No. 4 dumbbell-shaped specimen defined in JIS and a hysteresis loss was measured by the method described above in detail.

[0081] [Measurement of Universal Hardness]

[0082] A rubber thread which has not been wound around the center was drawn to have a whole length of 1000% and a universal hardness was measured by the method described above in detail.

[0083] [Measurement of Number of Breakage of Rubber Thread]

[0084] The number of breakages of the rubber thread which were generated until 100 cores were finished was counted. The result is shown in the following Table 1.

[0085] [Measurement of Resilience Coefficient of Core]

[0086] A hollow cylinder formed of aluminum having a weight of 200 g was caused to collide with the core obtained by the measurement of the number of breakages of the rubber thread at a speed of 40 m/s. The speed of the hollow cylinder before and after the collision and the speed of the core after the collision were measured. Thus, the resilience coefficient of the core was obtained in accordance with the law of momentum preservation. A mean value obtained by the measurement for 10 cores is shown in the following Table 1.

[0087] [Measurement of Amount of Compression and Deformation of Core]

[0088] First of all, an initial load of 98N was applied to the core obtained by the measurement of the number of thread cuts, and a load was gradually increased and a final load of 1274N was then applied. Thus, the amount of deformation of the core was measured from the application of the initial load to that of the final load. A mean value obtained by the measurement for 10 cores is shown in the following Table 1. TABLE 1 Result of Evaluation Compar- Compar- Example Example Example Example Example Example Example ative ative Comparative 1 2 3 4 5 6 7 Example 1 Example 2 Example 3 Synthetic Polyisoprene 70 85 85 85 85 85 85 70 50 70 Natural Rubber 30 15 15 15 15 15 15 30 50 30 Zinc Oxide 1 1 1 1 1 1 1 1 1 1 Antioxidant 1 1 1 1 1 1 1 1 1 1 Vulcanization 1.0 1.0 1.0 0.7 1.0 1.0 1.5 0.4 1.0 1.0 Accelerator D Vulcanization 0.5 0.5 0.5 0.2 0.5 0.5 0.5 0.1 0.5 2.0 Accelerator CZ Sulfur 3.0 2.0 3.0 3.0 4.0 1.5 3.0 3.0 3.0 1.0 Sulfur/Valcanization 2.0/1 1.3/1 2.0/1 3.3/1 2.7/1 1.0/1 1.5/1 6.0/1 2.0/1 0.3/1 Accelerator Hysteresis Loss (%) 29.5 26.7 25.3 24.2 22.4 28.2 27.8 29.4 37.3 35.3 Universal Hardness 2.94 1.99 2.52 2.16 2.99 1.58 3.32 1.33 4.71 3.24 (N/mm²) Crosslinking Density of 2.4 2.1 2.4 2.3 2.5 1.7 2.2 0.7 2.4 1.9 Polysulfide Bond * Number of Breakage of 0 3 1 1 0 5 0 53 0 0 Rubber Thread Resilisnce Coefficient 0.8014 0.8057 0.8076 0.8131 0.8203 0.8028 0.8040 0.8001 0.7942 0.7829 of Core Amount of Compressive 2.74 2.90 2.78 2.84 2.72 2.94 2.66 2.96 2.68 2.70 Deformation of Core (mm)

[0089] In the Table 1, the rubber thread is often broken in the comparative example 1 in which the universal hardness is small. In the comparative examples 2 and 3 in which the hysteresis loss is great, the resilience coefficient of the core is small. On the other hand, the rubber thread is less broken and the resilience coefficient of the core is great in each example. The high resilience of the core implies the high resilience of a golf ball obtained from the core. From the results of evaluation, the advantage of the present invention was confirmed.

[0090] The above description is only illustrative and various changes can be made without departing from the scope of the present invention. 

What is claimed is:
 1. A golf ball comprising: a center; a rubber thread layer formed by winding a rubber thread around the center; and a cover, wherein the rubber thread is formed of a material in which a hysteresis loss is 30% or less with an extension ratio of 0% to 1000% and a universal hardness of a DIN standard measured with a load of 3 mN is 1.5 N/mm² to 5 N/mm² when a whole length is increased up to 1000%.
 2. The golf ball according to claim 1, wherein a base rubber of the rubber thread contains synthetic polyisoprene and a ratio of the synthetic polyisoprene to the whole base rubber is 70% by weight or more.
 3. The golf ball according to claim 2, wherein a ratio of trans 1, 4 bond in the synthetic polyisoprene is 2% or more.
 4. The golf ball according to claim 1, wherein the rubber thread is crosslinked with sulfur and a vulcanization accelerator and a weight ratio of the sulfur to the vulcanization accelerator is 0.8/1 to 8/1.
 5. The golf ball according to claim 1, wherein a crosslinking density of polysulfide bond of the rubber thread is 1×10⁻⁸ mol/mm³ to 3×10⁻⁸ mol/mm³. 